1. The Field of the Invention
This invention relates to rotating wing aircraft, and, more particularly to rotating wing aircraft relying on autorotation of a rotor to provide lift.
2. The Background Art
Rotating wing aircraft rely on a rotating wing to provide lift. In contrast, fixed wing aircraft rely on air flow over a fixed wing to provide lift. Fixed wing aircraft must therefore achieve a minimum ground velocity on takeoff before the lift on the wing is sufficient to overcome the weight of the plane. Fixed wing aircraft therefore generally require a long runway along which to accelerate to achieve this minimum velocity and takeoff.
In contrast, rotating wing aircraft can take off and land vertically or along short runways inasmuch as powered rotation of the rotating wing provides the needed lift. This makes rotating wing aircraft particularly useful for landing in urban locations or undeveloped areas without a proper runway.
The most common rotating wing aircraft in use today are helicopters. A helicopter typically includes an airframe, housing an engine and passenger compartment, and a rotor, driven by the engine, to provide lift. Forced rotation of the rotor causes a reactive torque on the airframe. Accordingly, conventional helicopters require either two counter rotating rotors or a tail rotor in order to counteract this reactive torque.
Another type of rotating wing aircraft is the autogyro. An autogyro aircraft derives lift from an unpowered, freely rotating rotor or plurality of rotary blades. The energy to rotate the rotor results from a windmill-like effect of air passing through the underside of the rotor. The forward movement of the aircraft comes in response to a thrusting engine such as a motor driven propeller mounted fore or aft.
During the developing years of aviation aircraft, autogyro aircraft were proposed to avoid the problem of aircraft stalling in flight and to reduce the need for runways. The relative airspeed of the rotating wing is independent of the forward airspeed of the autogyro, allowing slow ground speed for takeoff and landing, and safety in slow-speed flight. Engines may be tractor-mounted on the front of an autogyro or pusher-mounted on the rear of the autogyro.
Airflow passing the rotary wing, alternately called rotor blades, which are tilted upward toward the front of the autogyro, act somewhat like a windmill to provide the driving force to rotate the wing, i.e. autorotation of the rotor. The Bernoulli effect of the airflow moving over the rotor surface creates lift.
Various autogyro devices in the past have provided some means to begin rotation of the rotor prior to takeoff, thus further minimizing the takeoff distance down a runway. One type of autogyro is the “gyrodyne,” which includes a gyrodyne built by Fairey aviation in 1962 and the XV-1 convertiplane first flight tested in 1954. The gyrodyne includes a thrust source providing thrust in a flight direction and a large rotor for providing autorotating lift at cruising speeds. To provide initial rotation of the rotor, jet engines were secured to the tip of each blade of the rotor and powered during takeoff, landing, and hovering.
Although rotating wing aircraft provide the significant advantage of vertical takeoff and landing (VTOL), they are much more limited in their maximum flight speed than are fixed wing aircraft. The primary reason that prior rotating wing aircraft are unable to achieve high flight speed is a phenomenon known as “retreating blade stall.” As the airframe of the rotating wing aircraft moves in a flight direction, rotation of the rotor causes each blade thereof to be either “advancing” or “retreating.”
That is, in a fixed-wing aircraft, all wings move forward in fixed relation, with the airframe. In a rotary-wing aircraft, the airframe moves forward with respect to the air. However, rotor blades on both sides move with respect to the airframe. Thus, the velocity of any point on any blade is the velocity of that point, with respect to the airframe, plus the velocity of the airframe. A blade is advancing if it is moving in the same direction as the flight direction. A blade is retreating if it is moving opposite the flight direction.
The rotor blades are airfoils that provide lift that depends on the speed of air flow thereover. The advancing blade therefore experiences much greater lift than the retreating blade. One technical solutions to this problem is that the blades of the rotors are allowed to “flap.” That is, the advancing blade is allowed to fly or flap upward in response to the increased air speed thereover such that its blade angle of attack is reduced. This reduces the lift exerted on the blade. The retreating blade experiences less air speed and tends to fly or flap downward such that its blade angle of attack is increased, which increases the lift exerted on the blade.
Flap enables rotating wing aircraft to travel in a direction perpendicular to the axis of rotation of the rotor. However, lift equalization due to flapping is limited by a phenomenon known as “retreating blade stall.” As noted above, flapping of the rotor blades increases the angle of attack of the retreating blade. However, at certain higher speeds, the increase in the blade angle of attack required to equalize lift on the advancing and retreating blades results in loss of lift (stalling) of the retreating blade.
A second limit on the speed of rotating wing aircraft is the drag at the tips of the rotor. The tip of the advancing blade is moving at a speed equal to the speed of the aircraft and relative to the air, plus the speed of the tip of the blade with respect to the aircraft. That is equal to the sum of the flight speed of the rotating wing aircraft plus the product of the length of the blade and the angular velocity of the rotor. In helicopters, the rotor is forced to rotate in order to provide both upward lift and thrust in the direction of flight. Increasing the speed of a helicopter therefore increases the air speed at the rotor or blade tip, both because of the increased flight speed and the increased angular velocity of the rotors required to provide supporting thrust.
The air speed over the tip of the advancing blade can therefore exceed the speed of sound even though the flight speed is actually much less. As the air speed over the tip approaches the speed of sound, the drag on the blade becomes greater than the engine can overcome. In autogyro aircraft, the tips of the advancing blades are also subject to this increased drag, even for flight speeds much lower than the speed of sound. The tip speed for an autogyro is typically smaller than that of a helicopter, for a given airspeed, since the rotor is not driven. Nevertheless, the same drag increase occurs eventually.
A third limit on the speed of rotating wing aircraft is reverse air flow over the retreating blade. As noted above, the retreating blade is traveling opposite the flight direction with respect to the airframe. At certain high speeds, portions of the retreating blade are moving rearward, with respect to the airframe, slower than the flight speed of the airframe. Accordingly, the direction of air flow over these portions of the retreating blade is reversed from that typically designed to generate positive lift. Air flow may instead generate a negative lift, or downward force, on the retreating blade. For example, if the blade angle of attack is upward with respect to wind velocity, but wind is moving over the wing in a reverse direction, the blade may experience negative lift.
The ratio of the maximum air speed of a rotating wing aircraft to the maximum air speed of the tips of the rotor blades is known as the “advance ratio. The maximum advance ratio of rotary wing aircraft available today is less than 0.5, which generally limits the top flight speed of rotary wing aircraft to less than 200 miles per hour (mph). For most helicopters, that maximum achievable advance ratio is between about 0.3 and 0.4.
At high speeds and high advance ratios, the flapping loads, lead-lag loads, and other loads exerted on the blades of a rotorcraft can be very large. The vibrational modes of the blade can also be complex and coincide with frequencies in the range of cyclic loading of the blades. Composite materials, such as carbon fiber, advantageously provide very high strength and stiffness and lightness of weight. However, conventional composite manufacturing methods are not suitable for achieving the complex geometry of rotor blade having the needed flexural and vibrational properties.
Composite materials typically include a high strength fiber, such as fiberglass or carbon fiber, embedded within a polymeric matrix material. The composition of composite materials from fiber and a polymeric matrix enables the formation of complex shapes using plies of fiber and resin applied to a mold or mandrel. The plies may be applied to the mold along with a semi-liquid resin or may be pre-pregnated with a resin that solidifies around the fiber prior to applying the plies to the mold. Pre-impregnated (“pre-preg”) plies may then be subsequently cured in order to first melt the resin and then cause the resin to cross-link and become rigid.
Composite materials, particularly carbon fiber composites, have very high strength due to the inherent properties of the carbon fiber. For this reason carbon fiber composites have come to replace steel and aluminum in many aeronautical applications due to their high strength to weight ratio. However, prior manufacturing processes for making composite parts are limited as to the complexity of the parts that may be manufactured. The curing process of parts made of pre-preg plies requires the application of appropriate amounts of heat and pressure to the assembled plies. If too little heat and/or pressure is applied, the resin will not adequately cross link and the plies of carbon fiber will not adhere to one another properly. If too much heat is applied or heat is applied for too long, the resin will over-cure and begin to degrade.
In prior processes, a part made of multiple pre-preg plies is cured by applying multiple plies or mats of pre-preg carbon fiber to a mold. The plies are then compressed by inserting them within a vacuum bag or applying an opposing mold. The assembly is then inserted within an autoclave heated to a suitable temperature in order to cause the resin coating the pre-preg fibers to melt and cure in order to form a matrix of resin spanning each of the plies and having the carbon fiber embedded therein.
Parts having varying thickness are not manufacturable with repeatable and uniform curing throughout using this prior method. Due to the uniform application of heat, thicker portions of the part will be under-cured and/or thinner portions of the part will be over-cured. Temperature gradients will exist within the part inasmuch as outer surfaces of the part will be at higher temperature than inner portions of the part for significant amounts of time during the curing process. Uniform application of heat to the combined plies and one or more molds also results in thermal expansion of the molds and a corresponding variation in mold geometry and pressure applied to the part.
Composite parts having large thicknesses, i.e., larger than 0.25 inches, are not readily manufactured using plies of pre-preg fiber according to prior methods. Curing of a laminate of multiple plies requires pressing the plies together and distributing of the resin uniformly throughout the laminate while the resin is liquid following melting and prior to cross-linking. In general, pressure is applied by an outer mold liner or vacuum bag pressing inwardly on the part. For thick laminates pressure hysteresis exists throughout the part, i.e., the pressure at different distances from the surface of the part is not uniform. As a result, resin flow throughout the part is not uniform and the inter-ply bonding between plies is likewise not uniform. These non-uniformities result in wrinkling of plies both within the plane of each ply and out of the original plane of each ply. The application of pressure also results in significant compression of the plies from their original thickness. During compression of thick laminates, the large compression distance may cause plies, or fibers within plies, to shift from their original positions, resulting in unpredictability and non-uniformity of part strength.
The effect of pressure hysteresis is exacerbated and compounded by the thermal gradients due to non-uniform thickness. The thermal gradients result in non-uniform resin viscosity and a corresponding increase in the non-uniform resin distribution. Non-uniform resin viscosity also results in non-uniform flow of resin, which increases in plane and out of plane wrinkling of the plies as well as increased porosity of the resin matrix. The presence of thermal gradients also causes stresses within the final part which may cause the part to deform from the dimensions of the mold.
The limitations of prior composite manufacturing processes make them unsuitable for manufacturing composite rotor blades, which generally have a large thickness at the root and a much smaller thickness along much of the blade. It would therefore be an advancement in the art to provide methods and apparatus suitable for manufacturing composite rotor blades having a root portion with a large thickness and a blade portion with a much smaller thickness.